Is there a way to use the normal of a face to model something else?
You can also select the face, then "view->align view to selected->front" (shift Numpad 1) and then change the 3d manipulator to "view" (instead of "global" or "local"). Then you can extrude a vertex along the normal by pressing "E YY" or "E XX" or use the manipulator to move along the aligned view axis.